EarthX requires we install an automatic over-voltage protection (crowbar) for alternator type charging systems.  According to EarthX, "In the event of a charging system failure where the voltage increases to above 15.5V, the resistance to charging current increases, and above 16V the charging current is completely blocked. The time delay for this feature is 1 second to allow the aircraft alternator’s over voltage protection (crowbar circuit) to activate first. This design offers charge voltage protection greater than 40V. The discharge current (current out of battery) is unaffected in this situation. EarthX requires having automatic over-voltage protection (crowbar) for alternator type charging systems (not required for <20 Amp pad mount standby alternators)."

We are looking to use a Over Voltage Module (OVM-14), but not sure of the best way to install it.  Using the Viking Wire diagram, I believe the OVM-14 may work if inserted as shown in the other diagram.  Any suggestions would be appreciated.

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Loren, not sure if this helps, but...

The way the crowbar OV module works is, in the event of an OV condition the module grounds out the alternator field wire, which causes the breaker for that wire to trip. This disconnects the alternator field wire from any voltage source which, in turn, shuts down the alternator output.

In your electrical system you should have a breaker that, when pulled, causes the alternator to cease output. You want to put the OV module on that wire (and also connected to ground) so that when the module senses OV it grounds out that wire and trips the breaker.

Thanks Ken, I meant to include the 3rd drawing, now posted, showing how we might add a alternator field relay to our circuit, but not sure if this will work.  As you can see, our way has the relay open the alternator field, not the alternator output.


I am unclear as to why EarthX requires automatic over voltage protection, since their circuitry disconnects battery charging at 16 volts. In the past, the primary reason for OV protection was to protect avionics. When we started putting lithium batteries in our airplanes, protecting the battery from over charging also became an issue. But with EarthX the battery is protected from high voltage by internal circuitry. Many of today's avionics are safe up to about 30 volts. It would be interesting to understand why EarthX "requires" OV protection. It seems to me that monitoring voltage (and shutting down the alternator if it runs away) should be sufficient.

What does Jan say? Does he use OV protection?

Ken, my understanding is that EarthX's built-in disconnect, with 1 second delay, will stop further charging, but that is to protect the battery, not the avionics.  Where a normal lead acid battery may absorb enough over voltage to give the pilot time to recognize the situation and shut off the alternator, it's unlikely with this type battery.  Therefore, EarthX requires a OVM, or at least recommends.  Remember, even with both batteries completely disconnected, the alternator still powers our aircraft, including the engine.  Also, once the alternator is off-line, the EarthX will continue to provide electricity until it's normally discharged.  I value Jan's input, but he's not the battery guy or electrician.

What I'm looking for is guidance to install the OVM to our electrical system.

Guess I'd know better how to install a OVM, if I understood how it worked internally.  

After several discussions with Ken Ryan, and at his suggestion, we are now looking to install an over-voltage module OVM.png  with one wire connected to the 5 amp breaker (between the breaker and alternator), and the other end to ground.  If it works as planned, the OVM will trip the alternator field breaker before the EarthX battery(s) over-voltage connection will trip. 

Until you understand the EXACT workings of the voltage regulator in your alternator, do nothing.

Will you be providing that information?

No,  I don't have it.  But for you to start digging into this without it makes no scene to me.  Also, I can tell you that the more junk you install into the electrical system, the worse it gets.  

I have no desire to add anything, or spend money unnecessarily, but it is EarthX that says to install an over-voltage protection for avionics.   Since our aircraft is totally dependent on electrical, it seems prudent to protect it as much as possible.  I posted here to learn how to install over-voltage protection properly, and hopefully with the least complexity and expense.  Who do I contact to learn about our voltage regulator?

The AeroElectric Connection book by Bob Nuckolls gives some information, but not for our alternator.  

EarthX tells us that the internal circuitry of their batteries can only protect up to 60 vols:


The reason for the over voltage protection is for your safety. This feature is required and done on all certified aircraft as standard. Over voltage protection is required if your aircraft has the ability to produce over 60V because the BMS can only protect up to 60V. If it goes above this, then the battery can go into thermal runaway. If you had a crowbar over voltage protection circuit, this would protect from this scenario. If your regulator and your over voltage crowbar protection fail, then you need to manually turn your alternator off. 


Most certified aircraft have alternators with external regulators. Many experimental aircraft have alternators with internal regulators. The folks on the Aeroelectrics forum point out that an internally regulated alternator can fail in such a way that interrupting the wire that comes from the ignition will not turn the alternator off:


Some internally regulated alternators have an ignition switch terminal that controls the alternator. But that input does not make and break the field circuit. All it does is command the internal regulator to shut off the field. A failed internal regulator could ignore that command and continue to supply excessive field current. The external voltage regulator has easily accessible wires and circuit protection. It is relatively easy to tap into the externally regulated alternator field circuit to add a switch or automatic over-voltage protection. The internally regulated alternator gets its field current from within the alternator. The externally regulated alternator gets its field current from the aircraft main power bus.


As Jan suggests, it would be good to know "everything" about your alternator. Unfortunately, that's probably not possible. Specifically it would be good to know if the alternator is capable of producing more than 60 volts and it would be good to know if the alternator can fail in such a way that interrupting the wire from the ignition switch will not turn it off. Good luck finding that information.

It is also true that "if it ain't there it can't fail" but you have an electrically dependent airplane so your electrical system better be fault tolerant. In this case you would be adding a relay and a voltage sensitive microswitch, either of which could fail (along with the crimps and connections you make to install them). You need to be certain that if anything fails, your system will tolerate it, meaning that your engine will keep turning. You also need to have a plan for recognizing and dealing with any failures.

I think there are arguments both for and against installing OV protection. Given the placement of the lithium batteries inside the cabin, I personally would favor having OV protection. On the other hand, if the batteries were not in the cabin, I would tend to lean against installing OV protection.

If you are going to install OV protection and you have an internally regulated alternator, it would be better to use a relay to interrupt the alternator output, rather than the wire from the ignition switch.

Just 2 cents, nothing more.

In addition to protecting the lithium battery, on airplanes powered by Viking engines OV protection might also be important to protect the ECU from an over voltage event? That would be a question for Jan. How tolerant is the ECU to an over voltage event?

Maybe the crowbar OV module could be inserted into the existing system something like this:



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